Optical disk device and information recording/reproduction method

ABSTRACT

An optical disk apparatus including: a light source; an objective lens for converging light emitted from the light source toward an optical disk; a first photodetection device for detecting reflected light from the optical disk and outputting a first signal; a signal processing section for receiving the first signal and generating a signal containing information recorded on the optical disk; a second photodetection device for detecting a portion of the light emitted from the light source and outputting a second signal; a light source driving section for receiving the second signal, and based on the second signal, driving the light source so that output power of the light source equals a target value; and an amplitude fluctuation detection section for detecting an amplitude fluctuation amount of the second signal, and if the amplitude fluctuation amount exceeds a predetermined value, changing driving characteristics of the light source driving section.

TECHNICAL FIELD

The present invention relates to an optical disk apparatus and aninformation recording/reproduction method using an optical diskapparatus.

BACKGROUND ART

In recent years, recordable optical disks which allow a large amount ofinformation to be recorded thereon, and optical disk apparatuses whichare compatible with them, are becoming prevalent. FIG. 12 schematicallyshows the structure of a conventional optical disk apparatus which onlyperforms reproduction or a conventional optical disk apparatus whichperforms recording and reproduction. Since an optical disk apparatuswhich performs recording is generally also capable of reproduction, anoptical disk apparatus which performs recording and reproduction willsimply be referred to as an optical disk apparatus which performsrecording, in the present specification.

In the conventional optical disk apparatus shown in FIG. 12, lightemitted from a laser 111 is converged by a collimating lens 102 so as totake a predetermined convergence state, and enters a polarization beamsplitter 103 (which may also be abbreviated as PBS). The polarizationbeam splitter 103 reflects the incident light so that a portion thereofwill enter a frontlight detector 112. Most of the incident light istransmitted through the polarization beam splitter, and enters aquarter-wave plate 104, where the polarization direction of the incidentlight is converted from linear polarization to circular polarization.

On a recording layer of an optical disk 101 which is rotated by aspindle motor 107, the light which has been transmitted through thequarter-wave plate 104 is converged by an objective lens 105, which isdriven by an actuator 106, so as to take a predetermined convergencestate.

The light which has been converged on the recording layer of the opticaldisk 101 is reflected from the recording layer, so that the reflectedlight enters the quarter-wave plate 104 via the objective lens 105. Thequarter-wave plate 104 converts the polarization direction of thereflected light from circular polarization to linear polarization. Thispolarization direction is perpendicular to the polarization direction ofthe light which is emitted from the laser 111, transmitted through thepolarization beam splitter 103, and travels toward the quarter-waveplate 104.

The light which has been transmitted through the quarter-wave plate 104enters the polarization beam splitter 103. As described above, thislight is perpendicular to the polarization direction of the light whichis allowed to be transmitted through the polarization beam splitter 103,and therefore is not transmitted through to the laser 111 side, but isreflected toward a photodetector 113.

FIG. 13A and FIG. 13B respectively show the general constitution of alight source driving section 120 and a signal processing section 121which are connected to the frontlight detector 112 and the photodetector113.

As shown in FIG. 13A, the light which is received by the frontlightdetector 112 is converted into an electrical signal, and is output as afrontlight signal to the light source driving section 120. Based on thefrontlight signal, the light source driving section 120 drives the laser111 in such a manner that the laser light emitted from the laser 111 hasa constant output power. For this purpose, the light source drivingsection 120 includes a laser power controller (hereinafter abbreviatedas LPC) 114 and a high-frequency module (hereinafter abbreviated as HFM)118. The LPC 114 extracts a low-frequency component from the frontlightsignal, and controls a driving current for driving the laser 111 so thatthe low-frequency component of the frontlight signal stays constant. TheHFM 118 subjects the driving current received from the LPC 114 to ahigh-frequency modulation, so that the laser 111 is driven with themodulated driving current.

On the other hand, as shown in FIG. 13B, the light received by thephotodetector 113 is converted into an electrical signal, and input tothe signal processing section 121 as an RF signal. The signal processingsection 121 includes a servo control section 117 and an RF detectionsection 116, and the RF signal is input to the servo control section 117and the RF detection section 116. Based on the RF signal, the servocontrol section 117 generates a focusing signal, a tracking signal, andthe like for moving the objective lens along the focusing direction andthe tracking direction. From the RF signal, the RF detection section 116generates a reproduced signal, which contains the user information,address information, and the like recorded on the optical disk 101.

The polarization directions of the quarter-wave plate 104 and thepolarization beam splitter 103 are designed so that almost all of thereflected light from the optical disk 101 enters the photodetector 113.In practice, however, due to variation in the amount of birefringence ata substrate which is provided on the recording layer surface of theoptical disk 101, variation in the optical characteristics andadjustment of the quarter-wave plate 104, the polarization beam splitter103, and the like, and fluctuation and variation in the wavelength ofthe laser 111, etc., the polarization direction of the polarization beamsplitter 103 and the polarization direction of the reflected light arenot completely perpendicular, so that there will be some light enteringthe laser 111 in an actual optical disk apparatus. This light isreferred to as “returned light”.

In general, returned light increases as the light emitted from the laser111 increases. However, depending on the phase difference between thelight emitted from the laser 111 and the reflected light from theoptical disk 101, the reflected light may be weakened due tointerference with the light emitted from the laser 111. In this case,conversely, returned light will decrease as the light emitted from thelaser 111 increases. The returned light to the laser 111 is absorbed ina semiconductor chip of the laser 111, thus contributing to theresonation of the laser 111, i.e., emission. For this reason, the laseremission efficiency increases in the presence of returned light, wherebythe output power is increased.

FIG. 14 is a graph showing relationships between the driving current andthe output power of a laser. In the figure, a solid line 61 shows arelationship in the case where the light amount of the returned light tothe laser is small, whereas a broken line 62 shows a relationship in thecase where the light amount of the returned light to the laser is large.

Via control of the LPC 114 utilizing the frontlight signal as describedabove, the output power of the laser 111 is adjusted so as to beconstant. Herein, output power means the outgoing light amount from thelaser.

When the optical disk apparatus reproduces information which is recordedon the optical disk 101, the light amount of the reflected light rapidlychanges while tracing on the recording marks, pits, spaces, and the likewhich are formed on the optical disk 101. As a result, the light amountof the returned light to the laser 111 also changes.

However, since the changes in the light amount of the reflected lightdue to recording marks and spaces occur sufficiently faster than thecontrol of the LPC 114, if a state in which there is a small amount ofreturned light (a white circle 63 in the graph) transitions to a statewhere there is a large amount of returned light (a black circle 64 inthe graph) due to a change in the light amount of the reflected light,the output power will change within a range sandwiched by the twoarrows. In other words, although the current which drives the laser 111does not change, the output power will change, that is, the emissionefficiency will change. Such fluctuations in the output power willhereinafter be referred to as scoop.

FIG. 15A and FIG. 15B show relationships between recording marks, an RFsignal, and a frontlight signal. As shown in FIG. 15A, when recordingmarks 131 and a space 132 are disposed on a recording track 130 of anoptical disk as in the figure, reflectance will decrease at therecording marks 131. However, in the absence of scoop, the frontlightsignal 134 stays constant. In other words, the output power of the laser111 does not change. As a result, as shown in the figure, an RF signal133 having a proper waveform is obtained.

On the other hand, as shown in FIG. 15B, if the light amount of thereturned light increases, the output power of the laser 111 fluctuatesin accordance with the change in the light amount of the returned light,thus causing scoop. As a result, the frontlight signal 136 alsofluctuates. Since fluctuation in the frontlight signal 136 due to scoopoccurs sufficiently faster than the control speed of the LPC 114, theLPC 114 does not control the driving current for the laser 111 inresponse to the fluctuation in the frontlight signal 136. Therefore, tothe RF signal 135, not only the fluctuation due to a change in thereflectance and phase of the recording marks 131 and the space 132, butalso the fluctuation in the emission efficiency due to scoop is applied.For example, when the peak intensity of the RF signal 135 is used as areference, due to a decrease in reflectance as well as a decrease inemission efficiency, the intensity becomes smaller in the regions of therecording marks 131. Therefore, the asymmetry and degree of modulationwill shift relative to those of the RF signal 133 in FIG. 15A. As aresult, the quality of the RF signal is degraded, and the reproductionjitter and error rate are deteriorated.

When performing a recording to an optical disk on an optical diskapparatus which performs recording and reproduction, it is necessary toform recording mark which satisfy a predetermined standard or reference,in order to guarantee a compatibility such that the recorded opticaldisk will permit correct reproduction also on another optical diskapparatus. Therefore, on such an optical disk apparatus, a predeterminedrecording pattern is first recorded onto the optical disk. Then, therecorded marks are irradiated with light, and the asymmetry and degreeof modulation of an RF signal which is obtained through reproduction areevaluated. Based on the evaluation result, the optical disk apparatusadjusts the laser power used for recording so that the formed recordingmarks satisfy a predetermined standard or reference.

At this learning, if the laser output power has fluctuations due toscoop, the RF signal obtained from recording marks which are formed inthe aforementioned manner cannot be correctly evaluated. If asymmetry islost, a write compensation learning: for adjusting for edge shifts atthe front end and rear end of a recording mark can no longer beperformed accurately, either.

In order to reduce such scoop which exerts unfavorable influences on RFsignal detection, for example, Japanese Laid-Open Patent Publication No.2001-189028 proposes increasing the reflectance on the outgoing face ofthe laser in order to reduce the amount of returned light to the laser.Moreover, Japanese Laid-Open Patent Publication No. 2001-143299discloses increasing reproduction power to suppress noise when a jitterdue to scoop increases during reproduction of an optical disk. Moreover,Japanese Laid-Open Patent Publication No. 05-217193 proposes varying theoscillation frequency and duty of an HFM based on the reproduced radiusof the optical disk, thus suppressing scoop.

However, according to the method of Japanese Laid-Open PatentPublication No. 2001-189028, the returned light may rather increase inthe case where the reflectance of the optical disk is greater than thatof the outgoing end face of the laser, thus deteriorating thereproduction jitter and error rate. Moreover, according to the method ofJapanese Laid-Open Patent Publication No. 2001-143299, it is necessaryto measure the jitter, and sufficient effects cannot be obtained in thecase where the reproduced signal has a poor signal quality.

Moreover, the light amount of the returned light to the laser 111 alsochanges due to causes other than tracing on recording marks, pits,spaces, and the like formed on the optical disk 101. For example, if theoptical disk 101 which is under reproduction is warped, the distancebetween the laser 111 and the recording layer of the optical disk 101will fluctuate. Therefore, the phase difference between the lightemitted from the laser 111 and the reflected light from the optical disk101 will fluctuate, thus resulting in changes in the intensity of thereturned light due to light interference. Such fluctuations in thereturned light also cause scoop, whereby the RF signal quality will bedegraded and the reproduction jitter and error rate will bedeteriorated.

DISCLOSURE OF INVENTION

In view of the aforementioned problems, the present invention aims toprovide an optical disk apparatus and an informationrecording/reproduction method which suppress deterioration in thereproduction jitter, error rate, and the like associated with scoop, andwhich makes it possible to obtain a high-quality reproduced signal.

An optical disk apparatus according to the present invention comprises:a light source; an objective lens for converging light emitted from thelight source toward an optical disk; a first photodetection device fordetecting reflected light from the optical disk and outputting a firstsignal; a signal processing section for receiving the first signal andgenerating a signal containing information recorded on the optical disk;a second photodetection device for detecting a portion of the lightemitted from the light source and outputting a second signal; a lightsource driving section for receiving the second signal, and based on thesecond signal, driving the light source so that output power of thelight source equals a target value; and an amplitude fluctuationdetection section for detecting an amplitude fluctuation amount of thesecond signal, and if the amplitude fluctuation amount exceeds apredetermined value, changing driving characteristics of the lightsource driving section.

In a preferred embodiment, the light source driving section includes acurrent control section for receiving the second signal and generating adriving current which is controlled so that the output power of thelight source equals the target value, and a high-frequency module formodulating the driving current with a predetermined frequency andoscillation power.

In a preferred embodiment, the amplitude fluctuation detection sectiondetects the amplitude fluctuation amount of the second signal, and ifthe amplitude fluctuation amount exceeds the predetermined value,changes a modulation frequency of the high-frequency module.

In a preferred embodiment, the amplitude fluctuation detection sectiondetects the amplitude fluctuation amount of the second signal, and ifthe amplitude fluctuation amount exceeds the predetermined value,changes an oscillation power of the high-frequency module.

In a preferred embodiment, the current control section generates thedriving current based on a predetermined frequency component of thesecond signal, and the predetermined frequency component isapproximately 1/10 or less of a frequency of the first signal.

In a preferred embodiment, the amplitude fluctuation detection sectiondetects the amplitude fluctuation amount of the second signal, and ifthe amplitude fluctuation amount exceeds the predetermined value,changes the target value in the current control section.

In a preferred embodiment, the amplitude fluctuation detection sectionreceives the first signal, and based on the first signal, detects anamplitude fluctuation amount of a component of the second signal that isin synchronization with the first signal.

In a preferred embodiment, the amplitude fluctuation detection sectionincludes a high-pass filter, and detects the amplitude fluctuationamount of the second signal having passed through the high-pass filter.

In a preferred embodiment, the amplitude fluctuation detection sectionchanges an oscillation power in accordance with the type of the opticaldisk.

An information recording/reproduction method according to the presentinvention is an information recording/reproduction method by an opticaldisk apparatus including: a light source; an objective lens forconverging light emitted from the light source toward an optical disk; afirst photodetection device for detecting reflected light from theoptical disk and outputting a first signal; and a signal processingsection for receiving the first signal and generating a signalcontaining information recorded on the optical disk, the informationrecording/reproduction method comprising: a step of detecting a portionof the light emitted from the light source and outputting a secondsignal; a step of receiving the second signal, and based on the secondsignal, driving the light source so that output power of the lightsource equals a target value; and a step of detecting an amplitudefluctuation amount of the second signal, and if the amplitudefluctuation amount exceeds a predetermined value, changing drivingcharacteristics in the step of driving the light source.

In a preferred embodiment, the step of driving the light source includesa step of receiving the second signal and generating a driving currentwhich is controlled so that the output power of the light source equalsthe target value, and a step of modulating the driving current with apredetermined frequency and oscillation power.

In a preferred embodiment, the step of changing the drivingcharacteristics detects the amplitude fluctuation amount of the secondsignal, and if the amplitude fluctuation amount exceeds thepredetermined value, changes a modulation frequency in the modulationstep.

In a preferred embodiment, the amplitude fluctuation detection sectiondetects the amplitude fluctuation amount of the second signal, and ifthe amplitude fluctuation amount exceeds the predetermined value,changes an oscillation power of the high-frequency module.

In a preferred embodiment, the step of driving the light source executesa step of generating the driving current based on a frequency componentof the second signal, the predetermined frequency component beingapproximately 1/10 or less of a frequency of the first signal.

In a preferred embodiment, the step of changing the drivingcharacteristics detects the amplitude fluctuation amount of the secondsignal, and if the amplitude fluctuation amount exceeds thepredetermined value, changes the target value in the step of generatingthe driving current.

In a preferred embodiment, the step of changing the drivingcharacteristics receives the first signal, and based on the firstsignal, detects an amplitude fluctuation amount of a component of thesecond signal that is in synchronization with the first signal.

In a preferred embodiment, the step of changing the drivingcharacteristics further includes a step of removing a low-rangecomponent from the second signal, and detects the amplitude fluctuationamount of the signal from which the low-range component has beenremoved.

In a preferred embodiment, the step of changing the driving,characteristics changes an oscillation power in accordance with the typeof the optical disk.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing a first embodiment of an optical diskapparatus according to the present invention.

FIG. 2 is a block diagram showing the constitution of a light sourcedriving section and an amplitude fluctuation detection section of theoptical disk apparatus shown in FIG. 1.

FIG. 3A and FIG. 3B show signal waveforms in the light source drivingsection. FIG. 3C shows a signal waveform in the amplitude fluctuationdetection section.

FIG. 4 is a block diagram showing a more detailed constitution of theamplitude detection section according to the first embodiment.

FIG. 5A to FIG. 5C show waveforms of signals at various points in theamplitude fluctuation detection section according to the firstembodiment.

FIG. 6A and FIG. 6B are figures schematically showing correspondencebetween recording marks on an optical disk and an RF signal and afrontlight signal.

FIG. 7 is a graph showing an exemplary relationship between the outputpower and the bit error rate.

FIG. 8 is a block diagram showing the constitution of the light sourcedriving section and the amplitude fluctuation detection section, whichare a main portion of the second embodiment of the optical diskapparatus according to the present invention.

FIG. 9 is a block diagram showing a more detailed constitution of theamplitude fluctuation detection section according to the secondembodiment.

FIG. 10A and FIG. 10B show signal waveforms at various points in theamplitude fluctuation detection section according to the secondembodiment.

FIG. 11A and FIG. 11B are diagrams schematically showing an example inwhich the oscillation frequency and oscillation power of ahigh-frequency module are changed.

FIG. 12 is a block diagram showing the constitution of a conventionaloptical disk apparatus.

FIG. 13A and FIG. 13B are block diagrams showing the constitution of alight source driving section and a signal processing section,respectively.

FIG. 14 is a graph showing relationships between the driving current andoutput power of a laser.

FIG. 15A and FIG. 15B are diagrams schematically showing correspondencebetween recording marks on an optical disk and an RF signal and afrontlight signal.

BEST MODE FOR CARRYING OUT THE INVENTION First Embodiment

Hereinafter, a first embodiment of the present invention will bedescribed with reference to the drawings. The present embodimentillustrates an optical disk apparatus which, especially in the presenceof a scoop that is in synchronization with an RF signal, suppressesdeterioration of the reproduction jitter, error rate, and the like, andprovides a high-quality reproduced signal. Compared to a scoop due toother causes, a scoop which, is in synchronization with an RF signal ismost influential on the fluctuation or deformation of the waveform ofthe RF signal.

FIG. 1 is a block diagram showing the first embodiment of an opticaldisk apparatus according to the present invention. The optical diskapparatus 200 is suitably used as an optical disk apparatus capable ofperforming recording or reproduction which supports, in addition tooptical disks such as DVD-RAMs and DVD-R/RWs, high-recording-densityoptical disks for which recording is performed by using laser light inthe ultraviolet region (e.g., about 405 nm).

The optical disk apparatus 200 comprises a laser 11, an objective lens5, a photodetector 13, a frontlight detector 12, a light source drivingsection 31, an amplitude fluctuation detection section 15, and a signalprocessing section 32. In addition, the optical disk apparatus 200preferably comprises a collimating lens 2, a polarization beam splitter3, a quarter-wave plate 4, an actuator 6, and a spindle motor 7.

The laser 11, which is used as a light source for recording andreproduction, emits laser light of a wavelength which is in accordancewith the type or standard of the optical disk. Although FIG. 1 onlyshows one laser 11, the optical disk apparatus 200 may comprise aplurality of lasers 11 and/or photodetectors 13 so as to support aplurality of different types of optical disks 1.

Light which is emitted from the laser 11 is converged by the collimatinglens 2 so as to take a predetermined convergence state, and enters thepolarization beam splitter 3. The polarization beam splitter 3 reflectsthe incident light so that a portion thereof will enter the frontlightdetector 12. Most of the incident light is transmitted through thepolarization beam splitter, and enters the quarter-wave plate 4, wherethe polarization direction of the incident light is converted fromlinear polarization to circular polarization.

On a recording layer of the optical disk 1 which is rotated by thespindle motor 7, the light which has been transmitted through thequarter-wave plate 4 is converged by the objective lens 5, which isdriven by the actuator 6, so as to take a predetermined convergencestate.

The light which has been converged on the recording layer of the opticaldisk 1 is reflected from the recording layer, so that the reflectedlight enters the quarter-wave plate 4 via the objective lens 5. Thequarter-wave plate 4 converts the polarization direction of thereflected light from circular polarization to linear polarization. Thispolarization direction is perpendicular to the polarization direction ofthe light which is transmitted through the quarter-wave plate 4 andtravels toward the objective lens 5.

The light which has been transmitted through the quarter-wave plate 4enters the polarization beam splitter 3, and is reflected toward thephotodetector 13.

The photodetector 13 functions as a first photodetection device. Thephotodetector 13 converts the received light into an electrical signal,and outputs it to the signal processing section 32 as an RF signal,which is a first signal. The signal processing section 32 includes aservo control section 17 and an RF detection section 16, and the RFsignal is input to the servo control section 17 and the RF detectionsection 16. Based on the RF signal, the servo control section 17generates a focusing signal, a tracking signal, and the like for movingthe objective lens along the focusing direction and the trackingdirection in order to radiate a light beam onto a track of the rotatingoptical disk in a predetermined convergence state. From the RF signal,the RF detection section 16 generates a reproduced signal, whichcontains the user information, address information, and the likerecorded on the optical disk 1.

The frontlight detector 12 functions as a second photodetection device.Light which is received by the frontlight detector 12 is converted intoan electrical signal, and is output to the light source driving section31 and the amplitude fluctuation detection section 15 as a frontlightsignal, which is a second signal. Since the light which is detected bythe frontlight detector 12 is a portion of the light emitted from thelaser 11, the light received by the frontlight detector 12 and thefrontlight signal are in proportion with the output power of the laser11.

Based on the frontlight signal, the light source driving section 31drives the laser 11 so that the output power of the laser light emittedfrom the laser 11 stays constant at a target value. Specifically, thelight source driving section 31 includes an LPC 14 and an HFM 18. TheLPC 14 extracts a low-frequency component from the frontlight signal,and controls a driving current for driving the laser 11 so that thelow-frequency component of the frontlight signal stays constant, thuspreventing the output power of the laser 11 from fluctuating from thetarget value. The HFM 18 subjects the driving current received from theLPC 14 to a high-frequency modulation, so that the laser 11 is drivenwith the modulated driving current.

The amplitude fluctuation detection section 15 detects the amplitude ofthe frontlight signal, and if the amount of amplitude fluctuationexceeds a predetermined value, changes the driving characteristics ofthe light source driving section 31. The driving characteristics to bechanged include the current for driving the laser 11, the modulationfrequency when performing high-frequency modulation, the oscillationpower, and the like. In the present embodiment, the returned light fromthe laser 11 changes in accordance with the recording marks, pits, orspaces on the tracks. In other words, the returned light is fluctuatingin synchronization with the aforementioned RF signal.

Therefore, due to influence of the returned light, the emissionefficiency of the laser 11 will change, so that amplitude fluctuationsassociated with the recording marks and spaces will occur in thefrontlight. Therefore, by detecting such amplitude fluctuations andchanging the driving characteristics of the light source driving section31 so that the amplitude fluctuations are reduced, the scoop in thelight emitted from the laser 11 is reduced.

Hereinafter, the light source driving section 31 and the amplitudefluctuation detection section 15 will be described in detail. FIG. 2 isa block diagram showing the specific constitution of the LPC 14 and theamplitude fluctuation detection section 15 of the light source drivingsection 31. The LPC 14 includes a low-pass filter (LPF) 14 a and acurrent control section 14 b. It is necessary that the LPC 14 hasresponse characteristics that are sufficiently slower than the frequencyof the RF signal which is obtained from the photodetector 13, so as notto react in response to the change in the intensity of the reflectedlight caused by the recording marks and spaces. For this reason, thelow-pass filter 14 a has characteristics such that a signal component ofa frequency which is sufficiently lower than the frequency of the RFsignal from the frontlight signal is allowed to be transmittedtherethrough, but that the high-range component, which is a frequencycomponent of the RF signal, is removed. For example, in the case wherethe optical disk is a DVD, a signal component having a frequency ofseveral tens of kHz or less is allowed to be transmitted.

FIG. 3A shows the waveform of a frontlight signal which is output fromthe frontlight detector 12. As described above, since the returned lightfluctuates in synchronization with the RF signal, the frontlight signalalso exhibits changes which are in synchronization with the RF signal.At a portion of the waveform indicated by an arrow 14 c, the signaloutput is lowered. This means that the average output power of the laser11 is lowered at the portion indicated by the arrow 14 c.

FIG. 3B shows the waveform of the frontlight signal having passedthrough the low-pass filter 14 a. As shown in FIG. 3B, the frontlightsignal having passed through the low-pass filter 14 a only exhibits alow-frequency component because the RF signal component, which is ahigh-frequency component, has been removed.

The current control section 14 b receives the frontlight signal, adjuststhe driving current for driving the laser 11 so that the low-frequencycomponent stays constant at a predetermined value, and outputs thedriving current to the HFM 18. Through this control, as shown in FIG.3B, the low-frequency component signal from the frontlight signal,indicated by the solid line, is brought up as shown by the arrow, thustaking a constant value shown by the broken line. Since the frontlightsignal is in proportion with the output power of the laser 11, thelow-frequency component of the output power of the laser 11 also staysconstant because the low-frequency component signal of the frontlightsignal is constant. Therefore, by determining the value to which thefrontlight signal is controlled so that the output power of the laser 11attains the target value, the laser 11 is controlled so as to emit lightwith an output power at the target value.

Note that, although the LPC 14 includes the low-pass filter 14 a in thepresent embodiment, the low-pass filter 14a can be omitted in the casewhere the frequency characteristics (response characteristics) of thecurrent control section 14 b are about the same as or lower than thoseof the low-pass filter 14 a. In other words, the LPC 14 may adjust thedriving current for driving the laser 11 by responding to the frontlightsignal at a frequency which is approximately 1/10 or less of thefrequency of the RF signal which is obtained from the optical disk 1.

In order to reduce the influence of scoop in the laser 11, the HFM 18modulates the received driving current with a high-frequency, thussuperposing a high-frequency AC current on the driving current, andapplies the modulated driving current to the laser 11.

The amplitude fluctuation detection section 15 detects an amplitudefluctuation amount of the frontlight signal, and if the amplitudefluctuation amount exceeds a predetermined value, changes the drivingcharacteristics of the light source driving section 31. For thispurpose, the amplitude fluctuation detection section 15 includes ahigh-pass filter 15 a and an amplitude detection section 15 b, receivesthe frontlight signal and passes it through the high-pass filter 15 a,and inputs the frontlight signal having passed through the high-passfilter 15 a to the amplitude detection section 15 b. The amplitudefluctuation detection section 15 detects fluctuations in ahigh-frequency component of the frontlight signal. For this purpose, thehigh-pass filter 15 a has characteristics such that it blocks or removesa low-range component, i.e., a signal which has a lower frequency thanthe frequency of the RF signal. In the case where the optical disk 1 isa DVD, the high-pass filter 15 a has characteristics such that it allowsany frequency higher than several hundreds of kHz to pass therethrough.FIG. 3C shows the waveform of a signal which is obtained by thefrontlight signal shown in FIG. 3A being passed through the high-passfilter 15 a. Although, as shown in the figure, the output is lowered onthe waveform of the frontlight signal at the portion indicated as 14 c,the frontlight signal having passed through the high-pass filter 15 aonly contains a high-frequency component, and the average fluctuation inthe output has been removed.

The amplitude detection section 15 b detects the amplitude fluctuationamount of the frontlight signal having passed through the high-passfilter 15 a, and changes the driving characteristics of the light sourcedriving section 31 if the amplitude fluctuation amount exceeds thepredetermined value. As shown in FIG. 4, the amplitude detection section15 b includes, for example, a divider 51, a low-pass filter 52, and acomparator 53.

The divider 51 receives the frontlight signal having passed through the-high-pass filter 15 a and the RF signal from the photodetector 13, anddivides the frontlight signal having passed through high-pass filter 15a by the RF signal. FIG. 5A and FIG. 5B show the waveforms of an RFsignal which is input to the divider 51 and an output signal from thedivider 51. The waveform of the frontlight signal having passed throughthe high-pass filter 15 a is shown in FIG. 3C. The frontlight signalhaving passed through the high-pass filter 15 a contains a signalcomponent which is in synchronization with the RF signal, and itsamplitude is lopsided toward the minus side due to scoop. Therefore, thesignal which is obtained through division has a waveform such that itsamplitude is greatly enlarged toward the minus side.

The low-pass filter 52 allows only the DC component of the signal whichis obtained through division to pass therethrough. In the case where theoptical disk 1 is a DVD, the low-pass filter 52 allows a signalcomponent of several hundreds of kHz or less to pass therethrough. FIG.5C shows the waveform of the signal having passed through the low-passfilter 52. Thus, the signal has a DC component which is offset towardthe minus side.

The comparator 53 compares this signal against a predetermined settingvalue, and if it exceeds the setting value, outputs a control signal tothe light source driving section 31 so as to change the drivingcharacteristics thereof. In the present embodiment, as shown in FIG. 2,a control signal is output to the current control section 14 b so as tochange its output power target value.

FIG. 6A and FIG. 6B show relationships between recording marks, the RFsignal, and the frontlight signal, in the cases where the output powerof the laser 11 is 0.5 mW and 1.0 mW. As shown in the figure, in thecase where the output power of the laser 11 is 0.5 mW, the frontlightsignal 24 is constant and the RF signal-has a proper waveform. On theother hand, in the case where the output power of the laser 11 is 1.0mW, the light amount of the returned light increases, and a scoop occursdue to the output power of the laser 11 fluctuating in accordance withchanges in the light amount of the returned light. Therefore, thefrontlight signal 26 fluctuates. Moreover, the RF signal 25 has smallerintensities in the regions of the recording marks 21.

In such a case, if the comparator 53 detects amplitude fluctuations inthe frontlight signal as shown in FIG. 5C, the current control section14 b lowers the output power target value so that the laser 11 is drivenwith a lower driving current. As a result, the light amount of thereturned light is reduced, the occurrence of scoop is suppressed, andthe fluctuations of the frontlight signal, i.e., the fluctuations of theoutput power of the laser 11, are suppressed.

FIG. 7 shows an error rate (Byte Error Rate, hereinafter abbreviated asBER) of the data which is obtained from the reproduced signal, in thecase where a scoop occurs with an increase in the output power of thelaser 11 as has been described with respect to FIG. 5A and FIG. 5B. If ascoop occurs, the symmetry of the RF signal is lost, and therefore thedata cannot be correctly reproduced and the error increases. As shown inthe figure, BER is 1×10⁻³ in the case where the output power of thelaser 11 is 1.0 mW, but is lowered to 1×10⁻⁴ when the output power islowered to 0.5 mW.

Thus, according to the present embodiment, in the case where the outputpower of the laser fluctuates due to returned light, the influence ofthe scoop is evaluated by using a frontlight signal, and if theinfluence of the scoop is large, the driving characteristics of thelight source driving section are changed, whereby the returned light canbe reduced. Therefore, the output power of the laser can be stabilized.Moreover, the influence of the fluctuation in the laser output power canbe removed from or reduced in the RF signal, and deterioration in thewaveform symmetry, reproduction jitter, and error rate can be prevented.

By using the present invention for an optical disk apparatus which iscapable of recording, it becomes possible to accurately perform learningof recording power and write compensation learning. Even in an apparatuswhich only performs reproduction, since the reproduction jitter anderror rate are reduced, the RF signal can be detected more accurately.

Moreover, according to the present embodiment, the reproduction jitterand error rate can be reduced without changing the laser structure ortaking jitter measurements.

Note that, in the present embodiment, a frontlight signal which haspassed through the high-pass filter 15 a is divided by the RF signal atthe amplitude detection section 15 b, and the amplitude fluctuationamount of the resultant signal is detected. This is in order to, in theamplitude fluctuation amount of the frontlight signal, only detect acomponent which is in synchronization with the RF signal, withoutdetecting any noise in the circuitry, which will not be in proportionwith the outgoing light and only appear in the frontlight signal. In thecase where a constitution that makes it possible to eliminate noise inthe circuitry is used, and an amplitude fluctuation amount containing acomponent which is in synchronization with the RF signal as well as acomponent which is not in synchronization with the RF signal is to bedetected, the constitution of a second embodiment described below can beutilized.

Second Embodiment

Hereinafter, a second embodiment of the present invention will bedescribed with reference to the drawings. The present embodimentillustrates an optical disk apparatus which, in the case where there isa scoop that is not in synchronization with the RF signal, suppressesdeterioration of the reproduction jitter, error rate, and the like, andprovides a high-quality reproduced signal. An example of scoop which isnot in synchronization with the RF signal may be, as described earlier,a scoop which occurs due to warpage of an optical disk. If a warpedoptical disk is subjected to reproduction, the distance between thelaser and the recording layer of the optical disk fluctuates, and thephase difference between the returned light and the light emitted fromthe laser also changes. Therefore, the intensity of the returned lightdue to light interference changes, and the output power of the laseralso fluctuates. FIG. 8 is a block diagram showing a main portion of thesecond embodiment of the optical disk apparatus according to the presentinvention. The optical disk apparatus of the present embodiment differsfrom the first embodiment in that an amplitude fluctuation detectionsection 15′ is comprised instead of the amplitude fluctuation detectionsection 15. The light source driving section 31 and other constituentelements are identical to those of the first embodiment.

As in the first embodiment, the amplitude fluctuation detection section15′ detects an amplitude fluctuation amount of the frontlight signal,and if the amplitude fluctuation amount exceeds a predetermined value,changes the driving characteristics of the light source driving section31. For this purpose, the amplitude fluctuation detection section 15includes a high-pass filter 15 a′ and an amplitude detection section 15b′, receives the frontlight signal and passes it through the high-passfilter 15 a, and inputs the frontlight signal having passed through thehigh-pass filter 15 a′ to the amplitude detection section 15 b′.

The amplitude fluctuation detection section 15′ detects fluctuation inthe high-frequency component of the frontlight signal which has not beencontrolled to a predetermined value by the LPC 14. For this purpose, thehigh-pass filter 15 a′ has characteristics such that it removes orsuppresses a low-range component, i.e., a signal which has a lowerfrequency than the frequency of a scoop that occurs due to warpage ofthe optical disk. The frequency of the scoop occurring due to warpage ofthe optical disk is determined from the degree of warpage of the opticaldisk, the wavelength of the light emitted from the laser 11, therotation speed of the optical disk, and the like. In the case where theoptical disk 1 is a DVD, the high-pass filter 15 a′ has characteristicssuch that it allows any frequency higher than several hundreds of kHz topass therethrough.

FIG. 10A schematically shows the waveform of a frontlight signal whichis influenced by a scoop occurring due to warpage of the optical disk.From the frontlight signal shown in FIG. 10A, the high-pass filter 15 aremoves or suppresses the low-range component, thus generating a signalwhich only contains the high-frequency component of the frontlightsignal, as shown in FIG. 10B. Since the DC component (which is alow-frequency component) is removed, a signal which fluctuates towardthe plus side and the minus side with respect to a reference potential(GND) is obtained.

For example, the amplitude detection section 15 b′ includes a comparator53′ as shown in FIG. 9, and compares the frontlight signal having passedthrough the high-pass filter 15 a′ against a predetermined settingvalue, and if it exceeds a setting value, outputs a control signal tothe light source driving section 31 so as to change the drivingcharacteristics thereof. In the present embodiment, as shown in FIG. 8,a control signal is output to the current control section 14 b so as tochange its output power target value. As in the first embodiment, in thecase where the scoop occurring due to warpage of the optical diskincreases with an increase in the output power of the laser 11, acontrol signal which reduces the output power target value is output tothe current control section 14 b. As a result, the returned light can bereduced, and the scoop occurring due to warpage of the optical disk canalso be reduced.

Thus, according to the present embodiment, in the case where there is ascoop due to warpage of the optical disk, too, the influence of thescoop is evaluated by using a frontlight signal, and if the influence ofthe scoop is large, the driving characteristics of the light sourcedriving section are changed, whereby reflected light from the laser andreturned light can be changed and reduced. Therefore, the output powerof the laser can be stabilized. Moreover, by stabilizing the outputpower of the laser, deterioration in the reproduction jitter and errorrate can be prevented.

Although the first and second embodiments illustrate examples where thescoop increases with an increase in the output power of the laser 11,the relationship between scoop and laser output power is not limitedthereto. If, owing to variations in the characteristics of thequarter-wave plate, polarization beam splitter, and like used in theoptical system as well as positioning variations, the light emitted fromthe laser and the reflected light from the optical disk are of such aphase difference relationship that they weaken each other, then thereturned light entering the laser will be reduced as the output power ofthe laser increases and the light amount of the reflected lightincreases. In such a case, the output power target value which is set inthe current control section 14 b may be controlled so as to increase.

Moreover, although the amplitude fluctuation detection section includesa high-pass filter in the first and second embodiments, the high-passfilter may be omitted because the LPC controls the laser output power soas not to fluctuate from a predetermined value at low frequencies, suchthat the low-frequency component of the frontlight signal issubstantially constant. In this case, since the output power is changedunder the control of the amplitude fluctuation detection section, andthe value of the low-frequency component of the frontlight signalchanges, it is preferable to change the reference against which theamplitude fluctuation amount of the frontlight signal is evaluated inthe amplitude detection section.

Moreover, the aforementioned relationship between the occurrence ofscoop and the output power of the laser 11 may differ depending on thecharacteristics of the optical system of the optical disk apparatus 200and the type of optical disk. Therefore, if the amplitude fluctuationamount exceeds a predetermined value when the amplitude fluctuationamount of the frontlight signal is detected, the target value to begiven to the current control section 14 b may be determined throughlearning. Specifically, when the amplitude fluctuation amount exceeds apredetermined value, a target may be initially set so as to lower theoutput power of the laser 11, and then it is checked whether theamplitude fluctuation amount decreases or not. If the amplitudefluctuation amount increases on the contrary, a target value which isgreater than the target value which was first set for the output powerof the laser 11 is set anew, and the amplitude fluctuation amount ischecked again. Such learning is repeated to find a target value to begiven to the current control section 14 b so as to reduce the amplitudefluctuation amount. In view of the possibility that the RF signal maydeteriorate due to an increased scoop and that data may not be correctlyobtained, it would be preferable to first make an adjustment by loweringthe output power, and if no improvement is observed in the amplitudefluctuation of the frontlight signal, then make an adjustment byincreasing the output power.

The amount of adjustment to be made in the output power when scoop isdetected is preferably set as follows. In the case where the opticaldisk 1 is a recordable optical disk, the upper limit value is preferablyset in a range such that the recording marks formed on the optical diskare not deteriorated by the reproduction light. For example, it is setto about 150% of the standard output power. The lower limit value ispreferably set in a range such that the signal noise removing ratio(Signal Noise Ratio, hereinafter abbreviated as SNR) are notdeteriorated in the pickup for receiving the signal and in the circuitsfor processing the signal from the pickup, and that focusing servo andtracking servo are not defeated. For example, it is set to about 50% ofthe standard output power.

In the first and second embodiments described above, a driving currentis chosen as the driving characteristics of the light source drivingsection, and the target value of the driving current in the currentcontrol section is changed. Alternatively, the oscillation frequency oroscillation power of the HFM 18 may be chosen as the drivingcharacteristics, and caused to change. FIG. 11A and FIG. 11B showwaveforms, respectively, in the case where the transmission frequency ischanged while keeping a constant output power, and in the case where theoscillation power is changed while keeping a constant transmissionfrequency. Depending on the type or standard of the optical disk, thethickness of the substrate which is provided on the surface of therecording layer may differ, and due to warpage (tilt) of the opticaldisk, the optical path length from the emission point of the laser tothe recording layer of the optical disk may change. Therefore, byadjusting the frequency and oscillation power of the HFM, the phasedifference between the returned light and the light emitted from thelaser changes, so that a setting where the laser power fluctuation isreduced can be obtained.

Note that, in the case where there is a tradeoff between the oscillationfrequency or oscillation power setting in the HFM 18 that most lowersthe scoop and the HFM setting that most reduces noise in the RF signal,e.g., in the case where increasing the HFM oscillation power will resultin a reduced scoop but an increased noise in the laser and other devicessuch that the SNR of the RF signal are deteriorated, it will benecessary to take both into accounts when setting the oscillationfrequency or oscillation power.

Moreover, the oscillation frequency or oscillation power of the HFM maybe changed depending on the optical disk which is to be subjected toreproduction. Since the amounts of reflectance and birefringence differdepending on the type of optical disk and the characteristics of each,the amount of returned light to the laser will change. Due to thisinfluence, the conditions under which scoop occur will change. In such acase, by changing the oscillation frequency or oscillation power of theHFM, the conditions under which scoop occur can be subdued, andconditions which will provide a good RF signal SNR can be set.

INDUSTRIAL APPLICABILITY

According to the present invention, deterioration in the reproductionjitter, error rate, and the like due to scoop can be suppressed, and ahigh-quality reproduced signal can be obtained. Therefore, the presentinvention can be suitably applied to an optical disk apparatus whichperforms recording or reproduction. In particular, the present inventionis suitably used for an optical disk apparatus which supports aplurality of types or a plurality of standards.

1. An optical disk apparatus comprising: a light source; an objectivelens for converging light emitted from the light source toward anoptical disk; a first photodetection device for detecting reflectedlight from the optical disk and outputting a first signal; a signalprocessing section for receiving the first signal and generating asignal containing information recorded on the optical disk; a secondphotodetection device for detecting a portion of the light emitted fromthe light source and outputting a second signal; a light source drivingsection for receiving the second signal, and based on the second signal,driving the light source so that output power of the light source equalsa target value; and an amplitude fluctuation detection section fordetecting an amplitude fluctuation amount of the second signal, and ifthe amplitude fluctuation amount exceeds a predetermined value, changingdriving characteristics of the light source driving section.
 2. Theoptical disk apparatus of claim 1, wherein the light source drivingsection includes a current control section for receiving the secondsignal and generating a driving current which is controlled so that theoutput power of the light source equals the target value, and ahigh-frequency module for modulating the driving current with apredetermined frequency and oscillation power.
 3. The optical diskapparatus of claim 2, wherein the amplitude fluctuation detectionsection detects the amplitude fluctuation amount of the second signal,and if the amplitude fluctuation amount exceeds the predetermined value,changes a modulation frequency of the high-frequency module.
 4. Theoptical disk apparatus of claim 2, wherein the amplitude fluctuationdetection section detects the amplitude fluctuation amount of the secondsignal, and if the amplitude fluctuation amount exceeds thepredetermined value, changes an oscillation power of the high-frequencymodule.
 5. The optical disk apparatus of claim 2, wherein the currentcontrol section generates the driving current based on a predeterminedfrequency component of the second signal, and the predeterminedfrequency component is approximately 1/10 or less of a frequency of thefirst signal.
 6. The optical disk apparatus of claim 3, wherein theamplitude fluctuation detection section detects the amplitudefluctuation amount of the second signal, and if the amplitudefluctuation amount exceeds the predetermined value, changes the targetvalue in the current control section.
 7. The optical disk apparatus ofclaim 6, wherein the amplitude fluctuation detection section receivesthe first signal, and based on the first signal, detects an amplitudefluctuation amount of a component of the second signal that is insynchronization with the first signal.
 8. The optical disk apparatus ofclaim 1, wherein the amplitude fluctuation detection section includes ahigh-pass filter, and detects the amplitude fluctuation amount of thesecond signal having passed through the high-pass filter.
 9. The opticaldisk apparatus of claim 1, wherein the amplitude fluctuation detectionsection changes an oscillation power in accordance with the type of theoptical disk.
 10. An information recording/reproduction method by anoptical disk apparatus including: a light source; an objective lens forconverging light emitted from the light source toward an optical disk; afirst photodetection device for detecting reflected light from theoptical disk and outputting a first signal; and a signal processingsection for receiving the first signal and generating a signalcontaining information recorded on the optical disk, the informationrecording/reproduction method comprising: a step of detecting a portionof the light emitted from the light source and outputting a secondsignal; a step of receiving the second signal, and based on the secondsignal, driving the light source so that output power of the lightsource equals a target value; and a step of detecting an amplitudefluctuation amount of the second signal, and if the amplitudefluctuation amount exceeds a predetermined value, changing drivingcharacteristics in the step of driving the light source.
 11. Theinformation recording/reproduction method of claim 10, wherein the stepof driving the light source includes a step of receiving the secondsignal and generating a driving current which is controlled so that theoutput power of the light source equals the target value, and a step ofmodulating the driving current with a predetermined frequency andoscillation power.
 12. The information recording/reproduction method ofclaim 11, wherein the step of changing the driving characteristicsdetects the amplitude fluctuation amount of the second signal, and ifthe amplitude fluctuation amount exceeds the predetermined value,changes a modulation frequency in the modulation step.
 13. Theinformation recording/reproduction method of claim 11, wherein theamplitude fluctuation detection section detects the amplitudefluctuation amount of the second signal, and if the amplitudefluctuation amount exceeds the predetermined value, changes anoscillation power of the high-frequency module.
 14. The informationrecording/reproduction method of claim 11, wherein the step of drivingthe light source executes a step of generating the driving current basedon a frequency component of the second signal, the predeterminedfrequency component being approximately 1/10 or less of a frequency ofthe first signal.
 15. The information recording/reproduction method ofclaim 11, wherein the step of changing the driving characteristicsdetects the amplitude fluctuation amount of the second signal and if theamplitude fluctuation amount exceeds the predetermined value, changesthe target value in the step of generating the driving current.
 16. Theinformation recording/reproduction method of claim 15, wherein the stepof changing the driving characteristics receives the first signal, andbased on the first signal, detects an amplitude fluctuation amount of acomponent of the second signal that is in synchronization with the firstsignal.
 17. The information recording/reproduction method of claim 10,wherein the step of changing the driving characteristics furtherincludes a step of removing a low-range component from the secondsignal, and detects the amplitude fluctuation amount of the signal fromwhich the low-range component has been removed.
 18. The informationrecording/reproduction method of claim 10, wherein the step of changingthe driving characteristics changes an oscillation power in accordancewith the type of the optical disk.